This invention relates to improved radiation-curable silicone compositions and to their use in a coating method. More particularly, this invention relates to compositions containing mercaptoalkyl functional siloxanes and alkenyl functional siloxanes which cure upon exposure to radiation.
The application of siloxane compositions to provide coatings of various substrates, for example, to improve the release of adhesive and other materials therefrom, has been practiced for many years. Although the conventional siloxane release coating compositions normally perform satisfactorily in use, there is a continuing demand for improved compositions. In particular a need has been expressed for compositions which cure faster and which require the application of less energy for the curing step. Also if such compositions can be applied in the absence of a solvent or other diluent, an additional advantage is realized.
Radiation curable coating compositions containing an organopolysiloxane bearing mercaptoorgano radicals and a polymethylvinylsiloxane curing agent are known from the disclosures of Michael, et al., U.S. Pat. No. 3,873,499; Gant, U.S. Pat. No. 4,064,027; and Bokerman, et al., U.S. Pat. No. 4,052,529. However, these compositions met resistance in the market place because they emitted an obnoxious odor after the cure process. The odor was quite noticeable, especially when the compositions were used in thin layer applications, such as in a paper coating process.
In order to reduce the odor associated with radiation cure of these types of coating compositions, Gordon, et al., U.S. Pat. No. 4,107,390, teach that the polymethylvinylsiloxanes in the compositions can be replaced by 1,1'-oxybis(1-methyl-1-silacyclopentene). Takamizawa, et al., G.B. Pat. No. 1,569,681, further teach UV curable compositions comprising (a) an organopolysiloxane having in the molecule at least one mercapto-containing organosiloxane unit, (b) an organopolysiloxane having in the molecule at least one allyl-containing organosiloxane unit, and (c) a photosensitizer. Although the odor associated with curing the compositions of these patents is not as marked as that with the vinyl-containing compositions, the unsaturated components used in these compositions are difficult and relatively expensive to prepare.
In still another attempt to reduce the odor associated with radiation cure of siloxane coating compositions, White, et al., copending U.S. patent application Ser. No. 713,940 which is owned by the assignee of this patent application, teach UV curable compositions comprising (a) mercapto group containing organosiloxane, (b) an organosiloxane having at least two siloxane units bearing cyclohexenylethyl substituents (C.sub.6 H.sub.9 CH.sub.2 CH.sub.2 --), and (c) a photoinitiator. However, there is still a need for siloxane coating compositions which will cure with lower levels of UV radiation and lower levels of electron beam radiation.
Oswald et al., U.S. Pat. No. 3,907,852, teach the selective monoaddition of silanes to .alpha., .omega.-dienes to form .omega.-alkenylsilanes which are then reacted with phosphine to form silylalkyl phosphines.
Hansen et al., U.S. Pat. No. 3,350,351, teach free radical vulcanization of a copolymer consisting of 40 mol percent 3-cyanopropylmethylsiloxane units, 58 mol percent of dimethylsiloxane units and 2 mol percent of 7-octenylmethylsiloxane units.